This invention relates to an improved parabolic current generator of a type suitable for providing convergence correction current for a color television display system.
U.S. Pat. No. 3,930,185 discloses a color television display system utilizing a picture tube including an electron beam gun assembly for producing three in-line beams. That patent discloses that in addition to the deflection yoke a quadrupole winding energized by parabolic current at the vertical deflection rate may be utilized to achieve horizontal convergence of vertical lines, thus simplifying the dynamic convergence apparatus which commonly requires both vertical and horizontal convergence correction currents.
A parabolic current generator suitable for providing the above described parabolic convergence correction current is disclosed in U.S. Pat. No. 3,911,295. The generator therein comprises a modified bridge circuit with two series-connected potentiometers coupled in parallel with the input terminals of the bridge and coupled to a sawtooth current source, the potentiometers serving to provide separate control of the bridge output current during positive and negative polarity portions of the input sawtooth current. It has been determined that under some circumstances the resistance of the potentiometers may undesirably change, resulting in an undesirable instability of the parabolic current produced by the bridge circuit.